The first known foray into the synthesis of poly(aromatic amines) dates back to 1834, when Runge oxidized aniline to an intractable green-black solid. The more severe the oxidation--typical oxidants included persulfate, dichromate, or chlorate compounds--the blacker the substance produced, and the less soluble the material was in concentrated acids. These compounds were tested as dyes, and by the 1870's the highly oxidized "Aniline Blacks" became the first members of the polyaniline family to gain industrial importance.
Researchers came to realize that the synthesized "polyaniline" (more appropriately, "polyanilines") exists in many different forms, depending upon the conditions of preparation. Some batches of an Aniline Black dye would undergo "green-ing" after a short time, while others retained their deep black color upon prolonged wear. Decades of research into the chemical structure of these dyes provided some insight into the phenomenon. The "ungreenable" Aniline Black--that is, the most highly oxidized Aniline Black--is chemically different insofar as a substantial number of chemically irreducible moieties are present in the polymer backbone, not convertible to the green form of the polyanilines, which consists of a linear indamine-type structure. Various idealized states of the polyanilines are depicted below: ##STR1## Emeraldine is blue in the basic form, but green in the acidic salt form. Perinigraniline is red in its pristine form, and is somewhat more bluish in its protonated form. The Aniline Black dyes obtained through severe oxidative polymerization of aniline are some combination of these idealized forms of polyaniline, including chemically irreducible moieties proposed as substituted phenazine rings. Willstatter and Moore demonstrated that through simple reduction-oxidation transformations these various oxidation states of the polyanilines are interconvertible ("Ungreenable" Aniline Black excepted). For example, leuco-Emeraldine can be oxidized to Emeraldine, and Emeraldine can be reduced to leuco-Emeraldine. Thus, the synthesis of any of these linear poly(phenyl amine) structures is a passport to the entire family of polyaniline polymers.
In 1985 it was reported that protonic acid doping of Emeraldine renders the material highly electrically conductive (A. G. MacDiarmid, J.-C. Chiang, W. Huang, B. D. Humphry, N. L. D. Somasiri, Mol. Cryst. Liq. Cryst. 125:309-318 (1985)). Emeraldine is prepared through a chemical or electrochemical polymerization of aniline under conditions similar to those used for Aniline Black. Cao et. al. (Y. Cao, A. Andreatta, A. J. Heeger, P. Smith, Polymer 30:2305 (1989)) determined optimum reaction conditions for the chemical synthesis as a function of a wide variety of synthetic parameters, including pH, relative concentrations of reactants, and polymerization time and temperature. Typically, the Emeraldine is prepared employing an aqueous suspension polymerization of aniline in a strongly acidic environment using strong chemical oxidants such as ammonium persulfate.
The poly(phenyl amines), and more specifically those polyanilines which are not highly over-oxidized, remain the most promising of conducting polymers, combining environmental stability, low cost of production, high conductivity, and a variety of chemically reversible oxidation and protonation states of different colors. Already batteries made from such materials are commercially available, as are blends for EMI shielding and anti-static applications.
Excepting F. Wudl's report of a multi-step synthetic preparation (Wudl et. al., Synthetic Metals 18:297 (1987)) which involves post-polymerization chemistry, the only reported polymerization route which yields a member of the family of polyanilines demonstrated to undergo reversible transformations of the various oxidized/reduced, protonated/de-protonated states has been through the oxidative polymerization of aniline in an aqueous acidic medium to form the material referred to as Emeraldine. Oxidation of the polymer occurs at a lower potential than that of the monomer, resulting in a tendency towards over-oxidation and a demonstrated propensity towards the formation of chemically irreducible units along the polymer backbone. It has been shown that exposure of Emeraldine to an aqueous acidic medium slowly degrades the properties of the material through side reactions (Y. Cao, A. Andreatta, A. J. Heeger, P. Smith, Polymer 30:2305 (1989))--hydrolysis of the quinoidal moieties which introduces oxygen into the polymer backbone has been demonstrated by several researchers (M. Angelopoulous, G. E. Asturias, S. P. Ermer, A. Ray, E. M. Scherr, A. G. MacDiarmid, Mol. Cryst. Liq. Cryst. 160:151 (1988))--which lower solubility in selected solvents and lower molecular weight. Thus, the Emeraldine is degraded by its own polymerization environment. The method of preparing this form of polyaniline is also disadvantageous. Since the conductive emeraldine salt so formed is insoluble, the oxidative polymerization of aniline in an aqueous medium affords only a heterogeneous route, requiring post-polymerization work-up prior to processing. To limit degradation during the chemical synthesis of polyaniline, conversion is often sacrificed by using a deficiency of oxidant. Side products of the polymerization reaction include unreacted aniline monomer (in salt form) and residual salts from the spent oxidant, complicating the industrial production process. The present invention involves a polymerization strategy which requires no oxidant, and resolves many of the drawbacks to the synthesis of polyaniline-type polymers by providing a bulk, homogeneous, cleaner polymerization route to electroactive polymers.